When shall we use the passive voice?

1.Use the passive voice when you want to emphasize the receiver,rather than the doer of an action. e.g. William led the invasion. The invasion was led by William. 2.Use the passive voice when you do not know the doer of an action. e.g. These books are written for teenagers. 3.Use the passive voice when you want to be polite, or when you do not want to identify the doer of an action. e.g. You are cordially invited to a party.